Currently, an optical lens module is widely used in an electronic product such as a videophone or a mobile phone's camera. In order to adjust, calibrate or detect the focusing operations of the optical lens module, the workers may manually rotate the optical lens module, which is engaged with a lens holding jag having an image sensor therein, to have the lens module rotate up and down along the lens' axis.
Referring to FIG. 1, a conventional focus adjustable system for generating an optical image having four corners' focus values is illustrated. In the focus adjustable system 10 of FIG. 1, an external light beam 11 passes through a lens surface 121 of a lens module 12, and is then projected onto an image sensor 14 equipped in a lens holding jag 13. After the light beam is imaged on the image sensor 14, an optical image I1 is outputted to a microprocessor 15. By a so-called full screen algorithm or a modulation transfer function (MTF) algorithm, the focus values UL1, UR1, DL1 and DR1 of the optical image I1 at four corners thereof are obtained. The external surface of the lens module 12 has a first engaging element (e.g. a screw thread) matching with the second engaging element 131 (e.g. a screw channel) on the inner surface of the lens holding jag 13, so that the lens module 12 may be rotated along the inner surface of the lens holding jag 13 forwardly or backwardly. As known, the first and second engaging elements may have any complementary structures as long as they match with each other, and are not to be redundantly described herein.
Referring to FIGS. 2(a) and 2(b), four original corners' focus values before the focus adjustment and four adjusted corners' focus values after the focus adjustment are respectively shown. In FIG. 2(a), the original focus values UL1, UR1, DL1 and DR1 before the focus adjustment are 45, 45, 45 and 45, respectively. In general, the focusing performance is proportioned to the focus value. That is to say, a lower focus value, e.g. 45, indicates the undesirable focusing performance. In order to the adjust the focus values, the worker may manually rotate the lens module 12, which is engaged with the lens holding jag 13, along the X-axis direction, i.e. the lens axis direction. After the focus values UL1, UR1, DL1 and DR1 are increased and reach acceptable values as shown in FIG. 2(b), e.g. 55 or above, the focusing performance of the lens module 12 is improved.
Unfortunately, during the focus adjusting process, the lens module 12 and/or the image sensor 14 is readily tilted. Under this circumstance, the optical path between the lens module 12 and the image sensor 14 will also be tilted. As shown in FIG. 2(c), due to this tilt phenomenon, the focus values UL1, UR1, DL1 and DR1 are changed to for example 50, 45, 30 and 41, respectively, which are not evenly distributed. Since these focus values are not evenly distributed, even if the worker manually rotates the lens module 12 along the X-axis direction, the adjusted focus values are still unevenly distributed. As known, this unsuccessful focusing adjustment may be caused by a fact that the lens holding jag 13 can move only along the X-axis direction. Moreover, since the tilt amount and the tilt angle are not clearly realized according to the prior art, the problem of generating uneven focus values is hard to be overcome.
In addition to the undesired tilted lens module 12 or image sensor 14, the inherent imaging tolerance of the lens module 12 during fabrication also contributes to a tilted optical path. The conventional focus adjustable method fails to discriminate whether the inherent imaging tolerance influences the focusing performance. Furthermore, the conventional focus adjustable method fails to evaluate the overall focusing performance after the lens module 12 is adjusted.
In views of the above-described disadvantages, the applicant keeps on carving unflaggingly to develop an improved focus adjustable method according to the present invention through wholehearted experience and research.